Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid catalyst. Similarly, cumene is an important precursor in the production of phenol and is produced by the alkylation of benzene with propylene in the presence of an acid catalyst.
Traditionally, ethylbenzene has been produced in vapor-phase reactor systems, in which the ethylation reaction of benzene with ethylene is carried out at a temperature of about 380-420° C. and a pressure of 150-250 psig in multiple fixed beds of zeolite catalyst. Ethylene exothermally reacts with benzene to form ethylbenzene, although undesirable chain and side reactions also occur. About 15% of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products. All these chain reaction products are commonly referred as polyethylated benzenes (PEBs). In addition to the ethylation reactions, the formation of xylene isomers as trace products occurs by side reactions. This xylene formation in vapor phase processes may yield an ethylbenzene product with about 0.05-0.20 wt % of xylenes. The xylenes show up as an impurity in the subsequent styrene product, and are generally considered undesirable.
In order to minimize the formation of PEBs, a stoichiometric excess of benzene, about 400-2000% per pass, is applied, depending on process optimization. The effluent from the ethylation reactor contains about 70-85 wt % of unreacted benzene, about 12-20 wt % of ethylbenzene product and about 3-4 wt % of PEBs. To avoid a yield loss, the PEBs are converted back to ethylbenzene by transalkylation with additional benzene, normally in a separate transalkylation reactor.
By way of example, vapor phase ethylation of benzene over the crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. No. 3,751,504 (Keown et al.), U.S. Pat. No. 3,751,506 (Burress), and U.S. Pat. No. 3,755,483 (Burress).
In recent years the trend in the industry has been to shift away from ethylbenzene vapor phase reactors to liquid phase reactors. Liquid phase reactors operate at a temperature of about 180-270° C., which is under the critical temperature of benzene (about 290° C.). One advantage of the liquid phase reactor is the very low formation of xylenes and other undesirable byproducts. The rate of the ethylation reaction is normally lower compared with the vapor phase, but the lower design temperature of the liquid phase reaction usually compensates economically for the negatives associated with the higher catalyst volume. In addition, the lower temperature liquid phase reaction enables a lower rate of the chain reactions that form PEBs; namely, about 5-8% of the ethylbenzene is converted to PEBs in liquid phase reactions versus the 15-20% converted in vapor phase reactions. Hence the stoichiometric excess of benzene in liquid phase systems is typically 150-400%, compared with 400-2000% in vapor phase.
Liquid phase ethylation of benzene using zeolite beta as the catalyst is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in alkylation/transalkylation reactions, especially to produce ethylbenzene and cumene. See, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); and U.S. Pat. No. 6,231,751 (ITQ-2).
Liquid phase aromatics alkylation plants offer significant advantages over vapor phase processes, because liquid phase processes operate at lower temperatures than their vapor phase counterparts. However, such liquid phase plants tend to be more sensitive to feed impurities which act as poisons to the zeolites used as alkylation and transalkylation catalysts. As a result most liquid phase processes require the use of high purity feedstocks and/or the provision of feed pretreatments to remove such feed impurities, particularly basic nitrogen compounds.
One known arrangement employed with liquid phase alkylation processes to remove feed impurities is the installation of a reactive guard bed located upstream of main alkylation reactor. The reactive guard bed incorporates one or more catalyst beds with the same or different catalysts, and it may be taken out of service at any time to replace catalyst, while the main alkylation unit continues to operate. In the reactive guard bed, the alkylatable aromatic compound and the alkylating agent are contacted in the presence of an alkylation catalyst prior to entry into the main alkylation reactor. The reactive guard bed not only serves to effect the desired alkylation reaction but also removes any reactive impurities in the feeds, such as nitrogen compounds, which could otherwise deactivate the remainder of the alkylation catalyst. The reactive guard bed catalysts are therefore subject to more frequent regeneration and/or replacement than the remainder of the alkylation catalyst. Also, the reactive guard bed is normally provided with a by-pass circuit so that the alkylation feedstocks can be fed directly to the alkylation reactor when the reactive guard bed is out of service. One example of an aromatics alkylation system including a reactive guard bed is disclosed in U.S. Pat. No. 6,995,295, the entire contents of which are incorporated herein by reference.
Although liquid phase alkylation processes produce much lower levels of polyalkylated species than vapor phase systems, process economics require the installation of a transalkylation reactor containing a transalkylation catalyst which converts polyalkylaromatic compounds in the presence of benzene to produce additional monoalkylated product. The benzene fed to the transalkylation reactor is typically a portion of the benzene recovered in the benzene column together with fresh make-up benzene, which is also fed to the column. All the remaining benzene recovered in the benzene column is fed through the reactive guard bed to the alkylation catalyst.
According to the present invention, an improved aromatics alkylation process has been developed in which the transalkylation reactor containing a transalkylation catalyst receives substantially all of the fresh make-up benzene, as compared to merely a slip stream from the benzene column overhead. Feeding all the make-up benzene to the transalkylation reactor allows the transalkylation reactor to be used as a reactive guard bed for removing impurities from the benzene feed. Also, it enables a much higher molar ratio of benzene to polyalkylated aromatic compounds to be maintained in the transalkylation reactor. This results in reduced polyalkylated aromatic by-product make, a higher per pass conversion of polyalkylated aromatic compounds and a higher thermodynamic yield of the desired monoalkylated product. With a higher per pass conversion of polyalkylated aromatic compounds, the recycle flow rates diminish and the amount of polyalkylated aromatic by-products requiring distillation also diminishes. Overall, energy costs are therefore reduced. In addition, the transalkylation reaction is thermo-neutral allowing the entire unit to be operated at relatively low temperatures. The transalkylation catalyst in the transalkylation reactor is generally a zeolite with higher aluminum content and a larger pore size than the alkylation catalyst. This greatly enhances the effectiveness of the transalkylation catalyst in reducing benzene feed impurities.
U.S. Pat. No. 5,902,917 discloses a process for producing alkylaromatic compounds, especially ethylbenzene and cumene, wherein a feedstock is first fed to a transalkylation zone and the entire effluent from the transalkylation zone is then cascaded directly into an alkylation zone along with an olefin alkylating agent, especially ethylene or propylene. However, the fresh make-up benzene is fed directly to the alkylation zone and there is no suggestion of using the transalkylation zone as a reactive guard bed.
In the improved process, the desired monoalkylated product is recovered from the effluents from the transalkylation and alkylation reactors and the unreacted alkylatable aromatic is fed to the alkylation reactor. In this way, loss of monoalkylated product to, for example, additional polyalkylated species in the alkylation reactor is avoided.
U.S. Pat. No. 6,096,935 discloses a process for producing alkylaromatic compounds using a transalkylation reaction zone and an alkylation reaction zone, wherein the transalkylation reaction zone effluent is passed to the alkylation reaction zone where aromatic compounds in the transalkylation reaction zone effluent are alkylated to the desired alkylaromatic compounds, particularly ethylbenzene and cumene. Again, there is no suggestion of using the transalkylation zone as a reactive guard bed and, although at least part of the fresh make-up benzene is fed to the transalkylation reaction zone, the entire effluent from the transalkylation zone is cascaded directly into the alkylation zone.
U.S. Patent Application Publication No. 2007/0179329 discloses an aromatics alkylation process in which the alkylatable aromatic compounds, and optionally at least part of the alkylating agent, are passed through a reactive guard bed and in the presence of a certain amount of water, containing alkylation or transalkylation catalyst, prior to entry into the alkylation zone.
U.S. Pat. No. 6,894,201 discloses a process and apparatus for removing nitrogen compounds from an alkylation substrate such as benzene. A conventional adsorbent bed containing clay or resin is used to adsorb basic organic nitrogen compounds, whereas a hot adsorbent bed of acidic molecular sieve is used to adsorb the weakly basic nitrogen compounds, such as nitrites, generally in the presence of water. The hot adsorbent bed can be provided in the transalkylation reactor upstream of the transalkylation catalyst (FIG. 6), in the alkylation reactor upstream of the alkylation catalyst (FIG. 7) or both (FIG. 8).